Timing belts have been formed by molding or casting an elastomer over tension members to form teeth and the body of the belt. Some timing belts are formed by attaching teeth to a thin metal or polymer band or by molding belts without internal tension elements. The pulley for driving such belts is a cylinder with grooves formed in the periphery to mesh with the teeth so as to allow the inside surface of the belt to ride on the periphery of the pulley. Examples of various types of belts and pulleys are shown in U.S. patents are U.S. Pat. Nos. 4,295,837; 4,218,932; 4,214,691; 4,193,527; 3,825,162; 4,614,508; and 4,453,660.
One problem with such belts is that they break long before they are expected to wear out in spite of being made from long wearing materials. The breaking invariably occurs in the relatively thin, flexible sections, sometimes called the web section, between the relatively thick and rigid toothed sections. It has been discovered that the breaking of the web is the result of fatigue caused by repeated bending of the belt as it wraps around the pulley during use. This problem is especially severe in tractor feed mechanisms used in high speed printers where the diameter of the pulley is relatively small and the peripheral speed is high and the belt is also being subjected to rapid tension and contraction forces as a result of rapid acceleration and deceleration of the pin belt. The cause of the problem was found to be that the contour of the belt as it wraps around the pulley is a series of arcs and straight sections instead of a constant radius. Because the periphery of the pulley is a circular cylinder with grooves, the arcs are not tangent to the straight sections and high bending stresses develop where the straight and curved sections meet. The resulting bending stresses cause belt failures. Prior attempts to solve the problem depended on use of pulley structures having raised projections between the grooves which raise the belt above the normal surface of the pulley. In addition to making a pulley structure more complicated, the belt is still subjected to being flexed as it goes through a series of straight and curved sections which can still produce fatigue. This is especially so with thin steel or plastic bands.